Humanity Has New 4.4 Million-Year-Old Baby Mama

As of today, humankind may have a new mother, and she looks nothing like we expected her to.

Described in a series of papers published Thursday in Science, Ardi — short for Ardipithecus ramidus — likely walked upright one million years before Lucy, the famous fossil skeleton whose species was regarded as the first member of the human lineage.

That position now belongs to Ardi, and the reconfiguration of our family tree is not merely cosmetic. Lucy’s story placed humanity’s origin on the savannah; Ardi took her first steps in the forest. From the shape of Lucy’s bones, scientists reasoned that the last common ancestor of humans and other great apes had resembled a chimpanzee; Ardi does not.

“This is a landmark,” said Dean Falk, a University of Florida evolutionary anthropologist who reviewed the findings. “The field will go into a frenzy.”

Falk’s assessment was echoed by paleontologists around the world, who have waited for 15 years since a handful of 4.4 million-year-old fossils, belonging to an unknown hominid species, were found in sediments along the Awash River in Ethiopia.

Even then, the fossils were clearly special. The name of the species, chosen by paleontologist discoverers Tim White, Gen Suwa and Berhane Asfaw of the Middle Awash Project, means “root ground ape” in local dialect. The fossils likely “represent a long-sought potential root species for the Hominidae,” they wrote in a 1994 Nature paper (.pdf).

From that original site, the Middle Awash team has since collected hundreds more A. ramidus fragments from 35 individuals, including a partially complete skeleton of the 4-foot-tall, 110-pound female now known as Ardi.

The researchers’ original assessment is not strictly correct: Ardi’s A. ramidus preceded Lucy’s Australopithecus afarensis, but some of her ancestors had already branched several million years earlier into a lineage that ends, for the moment, in chimpanzees and bonobos. But even if A. ramidus isn’t at the root of all hominids, it’s likely at the root of those hominids who became human.

Ardi “occupied the basal adaptive plateau of hominid natural history,” wrote the researchers in one of the Science papers, and “is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence.”

Lacking arches, and with thumblike big toes, A. ramidus had grasping feet still fitted for tree-climbing, but its pelvis appears suited to walking upright. And though A. ramidus‘ teeth were appropriate for eating fruits and leaves, they also display evidence of root and insect consumption. Ardi appears adapted to life both in the branches and on the ground.

That interpretation fits the environment implied by thousands of plant and animal fossils, as well as ancient soil deposits, also collected at the site. Most belonged to residents of woodlands, not grasslands: Whereas Lucy ventured onto the savannah, Ardi lived in a world of patchy, sun-dappled forests.

The savannah-as-cradle narrative is not the only conventional wisdom upset by Ardi. From Darwin on, most scientists thought that the last common ancestor of humans, chimpanzees and gorillas would be chimplike. The discovery that chimps share 99 percent of our DNA, and possess many of the skeletal features found in Lucy, supported this.

But A. ramidus lacks many typical features of chimpanzees, including large male canine teeth — a sign, say the researchers, that the ultra-aggressive social behaviors seen in chimpanzees were lost early in the human lineage. If so, male A. ramidus may have competed for female attention by bringing them food, rather than fighting each other. That could have contributed to the evolution of pair-bonding behavior, which later took the form of monogamous reproductive relationships. Ardi’s hands and pelvis were relatively humanlike; so, perhaps, was her heart.

All this suggests that chimpanzees and other great apes have changed far more than thought since we split from them, and are perhaps not the near-human analogues that scientists presumed.

“One effect of chimpanzee-centric models of human evolution has been a tendency to view Australopithecus as transitional between an apelike ancestor and early Homo. Ardipithecus ramidus nullifies these presumptions,” wrote C. Owen Lovejoy, a Kent State University anthropologist, in Science. “No ape exhibits an even remotely similar evolutionary trajectory to that revealed by Ardipithecus.”

William Jungers, a Stony Brook University paleoanthropologist, called the fossils “incredibly important.”

He disagreed with the researchers’ interpretation of A. ramidus‘ ability to walk upright — a skepticism seconded by Falk — but stressed the difference between this research and the hoopla that followed Ida, a 47 million-year-old lemur whose evolutionary importance was overhyped in May.

The fossils “will be intensely scrutinized and debated for years to come,” said Jungers. “The Ardipithecus saga impacts many aspects of human evolution in genuinely profound ways.”